A cushion of vehicle seat backs usually incorporates a frame structure for seat backs to provide the seat backs with sufficient strength and stiffness. There have been various measures to improve the seat backs in strength and stiffness.
JP 2006-81676-A discloses a frame structure improved in strength and stiffness by combining first and second metal frames with a hollow planar resin frame integrated with reinforcing ribs. JP 2005-194-A discloses a frame structure made of fiber reinforced resin provided with protrusive linear reinforcing parts which extend vertically at both sides of a plate-like framework main body. Such measures aim to improve the fixation strength of whole seat backs or to improve the strength and stiffness of a square frame structure for seat backs.
On the other hand, there are some frame structures for seat backs in which a seat frame is provided with a seatbelt retractor and a seatbelt guide (which may be called “seatbelt anchor”), independently from a vehicle frame. JP 10-129416-A discloses such a seat frame structure in which the seat back is improved in the attachment strength of the seatbelt retractor attached to the frame.
In a frame structure for seat backs in which a seatbelt retractor is attached to a seat frame, the seatbelt retractor and seatbelt guide are usually provided at the side of an end of the seat back in the vehicle width direction, and especially the seatbelt guide should be provided at the upper side of the frame. Because the seat belt is provided from the seatbelt retractor through the seatbelt guide to obliquely support a passenger, in case of a vehicle crash, the frame structure is subjected to a load applied forward or obliquely forward at the seatbelt guide installation section as the passenger moves forward according to the inertial law. Therefore, the stiffness of the frame structure for seat backs has to be enhanced enough to endure the bending and torsion caused by the load.
To improve the stiffness, JP 2006-81676-A and JP 2005-194-A disclose methods to enhance the strength and stiffness of a frame structure as a whole, and JP 2000-6696-A discloses a structure in which a deterrent bar is provided at the rear side of the seat back to achieve a high strength against a load applied toward the front side of the seat back. However, in such structures to improve the stiffness and strength, it might be difficult for the frame structure to be expected to be reduced in weight and otherwise might be expected to increase.
There are not many conventional suggestions to improve the stiffness of frame structures focusing on the load applied forward or obliquely forward through the seatbelt guide to the frame structures.
Accordingly, when focusing on the load applied forward or obliquely forward to the frame structure for seat backs, it could be helpful to provide a frame structure for seat backs capable of achieving the stiffness improvement of the frame structure effectively against the load while the frame structure is easily reduced in weight.